Billions of dollars are spent each year to control insect pests and additional billions are lost to the damage they inflict. Synthetic organic chemical insecticides have been the primary tools used to control insect pests but biological insecticides, such as the insecticidal proteins derived from Bacillus thuringiensis (B.t.), have played an important role in some areas. The ability to produce insect resistant plants through transformation with B.t. insecticidal protein genes has revolutionized modern agriculture and heightened the importance and value of insecticidal proteins and their genes.
Two different B.t. genes to be “stacked” so that a plant produces two different types of B.t. proteins. This has been done to increase the plant's spectrum of insect resistance and to prevent the development of insects that are resistant to a single type of B.t. protein. Compared to expressing a single gene, expressing multiple genes is relatively more involved. It is common in the generation of transgenic eukaryotes, including transgenic plants, that the coding regions for individual proteins are assembled and introduced as individual genes, with each having a separate set of promoter and transcriptional termination regions.
Toxin Complex (TC) proteins and genes, found primarily in bacteria of the genera Photorhabdus and Xenorhabdus but also in other bacterial genera such as Serratia, Pseudomonas, and Paenibacillus, are an important, relatively new source of insecticidal proteins and genes. There are at least three distinct classes of TC proteins. Native Class A TC proteins are approximately 280 kDa in size and possess insecticidal activity. Class B TC proteins (approximately 170 kDa) and Class C TC proteins (approximately 112 kDa) in combination enhance the insecticidal potency of Class A TC proteins but possess little to no insecticidal activity in the absence of a Class A TC protein. That is to say, Class B and Class C TC proteins in combination potentiate the insecticidal activity of Class A TC proteins. See e.g. US-2004-0208907 and WO 2004/067727 for a more detailed review of the art. Class A TC proteins possess insecticidal activity, alone, but this activity is relatively low. When a Class A TC protein is combined with a Class B and a Class C TC protein, they form a complex that is much more potent than the Class A TC protein alone.
The exact mechanism(s) of insecticidal action for TC proteins is not understood. It is possible that the proteins interact and/or assemble with each other during the course of killing the insect.